In order to manufacture ice-cream having proper softness, e.g. soft ice-cream, it is necessary to properly control the temperature of the refrigeration cylinder where a mix is whipped and cooled. Well known types of conventional controllers for such refrigeration are of either temperature-control type or torque control type. For example, Japanese Utility Model Early Publication No. 63-58586 discloses a temperature-control type controller which controls the temperature of the soft ice-cream based on indirectly detected temperature thereof. The temperature is measured from outside the cylinder containing the soft ice-cream, and is controlled by means of a thermostat, which turns off the refrigerator as it detects a preset OFF-temperature and turns on at a higher preset On-temperature, thereby maintaining the temperature of the soft ice-cream in the cylinder within these two temperatures.
The range of such temperatures may be varied so as to meet the temperature requirements for the ice-cream to be made of the mix in the cylinder. Similar approaches to such temperature control include detection of the temperature or the pressure of the refrigerant in the refrigerator.
A torque control, on the other hand, is based on the detection of the torque necessary to stir a properly frozen mix, as disclosed in Japanese Patent Early Publication No. 63-52843. An optimum torque for this purpose may be detected mechanically or electrically by measuring the electric power required by the beater. Through controlled stopping and restarting of the beater, an ice-cream mix of desired softness may be obtained. The range of the torque for such softness control may be varied for different types of ice-cream to be manufactured.
However, the former temperature control is subject to undesirable over-refrigeration caused by fluctuations in the behavior of the thermostat or erroneous temperature setting. This may happen since the stopping of the refrigeration depends on the preset OFF-temperature, and causes the ice-cream to be refrigerated too hard and a waste of electric energy.
On the other hand the latter temperature control based on the torque measurement may result in an excessive beating of the mix, which may destroy the texture of the ice-cream and make the ice-cream "too soft" to serve in a cup with good shape.
Thus, conventional approaches to the control in manufacturing ice-cream are not satisfactory.
As shown in FIG. 18, the period required for freezing or refrigerating the mix generally depends on the temperature of the soft ice-cream to be made, and so does the power to be input to the beater motor on the refrigeration period. It may be seen in this FIG., however, that the temperature of the ice-cream and the beater motor input vary little with refrigeration time after a certain period. This is due to the fact that the ice-cream attains a certain hardness as it is refrigerated, and exhibits an increased beating load which will balance out the refrigeration power input.
As mentioned above, ice-cream will become too soft under excessive beating. Consequently, refrigeration must be controlled so as not to be given continuously over a long period. This implies that the temperature control or torque control must be given in such a way as to permit intermittent refrigeration, which may be easily attained for a particular ice-cream if the range of the control temperature or torque is narrowed. However, since an ice-cream manufacturing apparatus must be designed for use with different kinds of ice-creams, the temperature or the torque cannot be limited in a narrow range. As a result if the temperature preset by the user is not appropriate, continuous or excessive beating may result and extremely degrade the ice-cream.
There have been known so-called twin freezers, which are capable of manufacturing more than two kinds of ice-creams. An example of this, as disclosed in Japanese Patent Early Publication No. 47-6681, has two refrigeration cylinders, left and right ones, each having a plunger for extracting the ice-cream therein. The freezer further has another plunger connected to both cylinders for extracting mixed ice-cream.
At the end of sale, ice-cream manufacturing apparatuses of this kind require daily pastaurization of the components which are in contact with the mix. Pasteurization may be made by injecting water or hot water containing a pasteurizing agent into the mix tanks and the refrigeration cylinders. Instead of such pasteurizing agent, recent pastaurization utilizes direct heating of the mix tanks and the refrigeration cylinders, as explained below.
For this purpose the refrigeration system having a compressor, a condenser, a decompressor, and an evaporator for refrigerating the ice-cream mix in a refrigeration cylinder and a mix tank, is operated as a heat pump, i.e. it is operated in a reverse direction by means of a four-way valve so as to make the evaporators function as radiators giving off heat supplied by the hot and highly pressurized refrigerant gas generated by the compressor. And the heat given off to the mix tank and the refrigeration cylinders, as disclosed in Japanese Utility Model Early Publication No. 63-20304. In a so-called hot gas system, the hot pressurized refrigerant gas supplied from the compressor is fed directly through bypassing nozzles into the evaporators, as disclosed in Japanese Patent Early Publication No. 60-46942.
Pasteurization of the twin type apparatuses is basically possible by means of such reversible refrigeration units as taught in Japanese Utility Early Publication No. 63-20304.
However, in the case of twin type apparatus a problem arises that if the two refrigeration units are heated independently for pasteurization as taught in prior art, the central plunger connected with both cylinders may not be heated to a pasteurizing temperature and not pasteurized adequately when the cylinder on one side is undergoing refrigeration, though the other cylinder is in pasteurization.
Ice-cream manufacturing apparatuses need periodic and interim cleaning and pasteurization at times, for example, just before holidays. In order to make the ice-cream softer and easy to discharge it out of the cylinders during such cleaning the refrigeration cylinders must be heated.
The cylinders are also heated to heat the ice-cream so that it recovers proper softness when it is frozen too hard, and to "regenerate" the ice-cream in which the ice-cream is heated once and then re-freezed to recover proper hardness lost in excessive beating. For these purposes some ice-cream manufacturing apparatuses, as one disclosed in Japanese Patent Early Publication No. 63-196232, are equipped with defrosting switches which enable the refrigeration unit to generate hot refrigerant gas to heat the cylinders to predetermined temperatures, and some, as one disclosed in Japanese Patent Early Publication No. 56-7656, are equipped with timers for controlling the period of heating the cylinders with hot gases.
However, the former Publication (JPEP No. 63-196232) resorts to only temperature control, so that excessive heating may happen on account of inaccuracy or malfunction of the temperature sensor of the apparatus, resulting in the degeneration of the ice-cream. The latter Publication (JPEP No. 56-7656) relies on the timer setting made by the user. However, since an optimum time setting is very difficult, over-defrosting or insufficient defrosting may easily take place.
The conventional heating system has another disadvantage in that at least two temperature sensors are required to detect the different temperatures and a rather complex electronic devices associated with the sensors. This is because the mix in the cylinders require different temperature depending on whether it is in the normal operating condition, process of removal from the cylinder, or regeneration process. (Removal temperature is the highest.)